Spin and recombination dynamics of excitons and free electrons in p-type GaAs : effect of carrier density

TL;DR

This study investigates how carrier density influences spin and recombination dynamics of excitons and free electrons in p-type GaAs at low temperature, revealing different spin relaxation mechanisms at varying excitation powers.

Contribution

It provides new insights into the effects of carrier density on spin relaxation and recombination pathways in p-GaAs, highlighting the transition from excitonic to free carrier regimes.

Findings

Spin lifetime is shorter for electrons recombining via e-A0 transition.

Excitation power increase causes a crossover to band-to-band emission.

Spin relaxation time decreases dramatically at high carrier densities.

Abstract

Carrier and spin recombination are investigated in p-type GaAs of acceptor concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly neutral and photoelectrons can either recombine with holes bound to acceptors (e-A0 line) or form excitons which are mostly trapped on neutral acceptors forming the (A0X) complex. It is found that the spin lifetime is shorter for electrons that recombine through the e-A0 transition due to spin relaxation generated by the exchange scattering of free electrons with either trapped or free holes, whereas spin flip processes are less likely to occur once the electron forms with a free hole an exciton bound to a neutral acceptor. An increase of exci- tation power induces a cross-over to a regime where the bimolecular band-to-band (b-b) emission becomes more favorable due to screening of the electron-hole Coulomb interaction and ionization of excitonic complexes and free excitons. Then, the formation of excitons is no longer possible, the carrier recombination lifetime increases and the spin lifetime is found to decrease dramatically with concentration due to fast spin relaxation with free photoholes. In this high density regime, both the electrons that recombine through the e-A0 transition and through the b-b transition have the same spin relaxation time.